Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Thallium (Greek thallos, “young shoot”), symbol Tl, soft, malleable metallic element that acquires a bluish-gray color upon exposure to the atmosphere. Thallium is in group 13 (or IIIa) of the periodic table. The atomic number of Thallium is 81.

Thallium was discovered spectroscopically in 1861 by the British chemist Sir William Crookes. It was isolated by Crookes and, independently, by the French chemist Claude August Lamy in 1862.


Thallium forms a hydroxide in water, and is soluble in nitric acid and sulfuric acid. It forms two series of salts, represented by thallous chloride, TlCl, and thallic chloride, which are univalent and trivalent, respectively. Thallous oxide, a black solid that, when molten, attacks glass and porcelain, is made by heating thallium in air at very high temperatures. Thallium has a high index of refraction and is therefore important in the manufacture of several types of optical glass.

Thallium ranks 60th in abundance among the elements in the crust of the earth and is a member of the aluminum family of metals. Thallium melts at about 304°C (about 579° F), boils at about 1457°C (2665° F), and has a specific gravity of 11.85. The atomic weight of thallium is 204.38. Thallium occurs in combination with pyrites, zinc blende, and hematite and is often recovered from the flue dust produced by pyrites ovens in which sulfur and iron are separated. Occasionally it is extracted from the mud produced in lead chambers used in manufacturing sulfuric acid.


Thallium sulfate, which is odorless, tasteless, and very poisonous, is used to exterminate rodents and ants. Thallium-activated sodium iodide crystals mounted in photomultiplier tubes are used in some portable scintillation counters to detect gamma radiation. The abilities of thallium bromoiodide crystals to transmit infrared radiation and of thallium oxysulfide crystals to detect the same radiation have been employed extensively in military communication systems. Thallium alloyed with mercury forms a fluid metal that freezes at -60° C (-76° F); it is used in low-temperature thermometers, relays, and switches. Thallium salts, which burn with a bright green flame, are used in rockets and flares.